An electric connection box described in Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2005-295724) is conventionally known. The electric connection box comprises a case accommodating a relay module having a relay mounted on a circuit board. When current is passed through the relay, the relay generates heat. The heat is dissipated into the case and then to the exterior through a window of the case.
In recent years, an increase in density has been demanded for electric connection boxes. This has resulted in a tendency to increase the number of relays accommodated in a case and thus the total quantity of heat generated by the relays. A size reduction has also been demanded for the electric connection boxes. The volume of the case thus tends to decrease. Thus, the heat generated by the relays may remain in the case, which may become hot. The increased temperature of the interior of the case may degrade the performance of electronic components mounted on a circuit board.
It is thus possible to form a suction port and an exhaust port in a case so that air can flow through the case, to cool the case by air. However, this technique cannot inhibit the relays from becoming locally hot. The reason is as follows. When current is passed through the relay, components of the relay such as a coil which are accommodated in a housing for the relay generate heat. In particular, the vicinity of the bottom of the housing where the relay components are arranged is likely to become hot. Thus, even when the housing for the relay is entirely cooled by air, air is prevented from flowing through the area between the bottom of the relay and the circuit board. This makes it difficult to inhibit the bottom of the relay from becoming locally hot. If the relay is mounted on the circuit board, heat is prevented from being transmitted to the circuit board made of a synthetic resin, which has a relatively low thermal conductivity, causing more heat to remain at the bottom of the relay.
It is thus possible to dispose a bus bar on the circuit board and to connect the bus bar to the relay so that heat generated by the components of the relay can be transmitted to the bus bar, which is then cooled by air. However, even with this technique, the air contacts only the surface of the bus bar, preventing heat from being efficiently transmitted from the bus bar to the air. This makes it difficult to sufficiently cool the relay.